Method for purifying bismuth



Patented Feb. 29, 1944 METHOD FOR PURIFYING BISMUTH Leon Jean Baptiste Guy-ard, Paris, France;

vested in the Alien Property Custodian No Drawing. Application September 5, 1941, Serial No. 409,751. In France September 12, 1940 6 Claims.

Some metallurgical treatments-namely, those concerning the metallurgy of copper, lead, silver and gold-give, as a subproduct, bismuth which is combined with important proportionsv of lead and sometimes with arsenic and traces of sulphur, copper, etc. An average composition :of these alloys is, for example:

Per cent Bismuth '70-'75 Lead 25-30 Arsenic 1-2 sists essentially to treat the previously pulverized alloy with a quantity of diluted nitric acid which exceeds slightly the quantity theoretically required to transform into nitrate all the lead of the alloy, this treatment being prolonged until all the lead is carried into solution in the form of nitrate and until the part of bismuth which was firstly transformed into nitrate is precipitated in the state of subnitrate.

In the practical realization of the invention diluted nitric acid is used in an excess of about 10 to 12% of the theoretical quantity corresponding to the lead of the alloy to be treated.

The treatment is advantageously executed at roomtemperature. Firstly, a brisk attack occurs. which transforms the main part of the lead and some quantity of the bismuth into nitrates which go into solution, and afterwards the attack slows down. The mass is then submitted to a mechanical stirring, the contact being prolonged during several days in order to insure a complete elimination of the lead which has not been firstly attacked. This lead is transformed into nitrate to the detriment of the bismuth nitrate which has been formed and is dissolved by degrees with the attendant precipitation of metallic bismuth.

Finally, when the prolongation of the attack has nearly annulled the initial acidity of the solution, the bismuth which remains in the form of a nitrate is precipitated in the state of subnitrate. When the precipitation is finished, a mixture of metallic bismuth (and eventually arsenic) and bismuth subnitrate is formed, with a solution of lead nitrate in which are only traces of bismuth nitrate which may at their turn be precipitated by dilution.

All the lead of the alloy treated is thus eliminated, but the solid mass which is formed by the mixture :of metal and bismuth su'bnitrate contains eventually the totality of the arsenic.

The method generally used for the elimination of this element consists in an oxidizing fusion of the metal, with addition :of an alkaline nitrate, for example, sodium nitrate. According to the invention, the bismuth .subnitr-ate present in the mass is advantageously used as oxidising agent. In fact this :salt begins its decomposition at C. with production .of nitrous products which react upon the arsenic and transform it into arseni'ous zanhydride which is volatile.

The reaction may be written in its simplest form:

From this reaction, it may be seen that every 1 percent in weight of As requires 2.8% of bismuth in the state of subnitrate.

This reaction is wholly realized at a temperature below the nascent red. The bismuth oxide thus formed remains as a. slag which separates from the molten metallic bismuth and can be collected separately for the purpose of recovering the bismuth by a subsequent reducing melting operation. The volatile arsenious anhydride which evolves during the reaction can be collected and condensed by any convenient known process.

As hereabove explained, amongst other advantages, the method according to the present invention does not necessitate the use of a special apparatus or of excessive quantities of reagent. The only subproduct obtained consists in lead nitrate.

Having thus described my invention, what I claim as new, and desire to secure by Letters Patent is:

l. The method for the recovery of bismuth I from its alloys containing lead and arsenic which comprises treating the pulverized alloy with a quantity of dilute nitric acid slightly in excess of that theoretically necessary to convert the lead content into nitrate, continuing this treatment for a period which will permit the dissolution of the lead content as a nitrate and the precipitation of only a minor proportion of the bismuth content as subnitrate, thereby forming a residual material containing the major portion of the bismuth in the metallic state, bismuth subnitrate and the arsenic content of the initial alloy, and subjecting said residual material to treatment for the recovery of bismuth.

2. The method as in claim 1, wherein the residual material from which the lead nitrate solution has been removed, and containing metallic bismuth, bismuth subnitrate, and arsenic, is subjected to an oxidizing fusion for eliminating the arsenic content, the bismuth subnitrate formed in said method serving as the oxidizing agent.

3. The method for recovering bismuth from metallurgical substances containing the same along with lead and arsenic which comprises treating said metallurgica1 substanc with dilute nitric acid in quantity suflicient only to dissolve the lead content and to convert a minor portion of the bismuth to a precipitate of bismuth subnitrate, separating the residue containing metallic bismuth, bismuth subnitrate, and arsenic, and subjecting said residue to an oxidizing fusion for removing the arsenic content.

4. In the method for recovery of bismuth from metallurgical substances containing the same along with lead and arsenic, the steps which comprise treating said metallurgical substance with a nitrate solution slightly in excess of that quantity necessary to dissolve the lead as nitrate and to form a minor proportion of bismuth subnitrate, and subjecting the residual material containing metallic bismuth, bismuth subnitrate and the arsenic content to an oxidizing fusion, wherein said bismuth subnitrate is the oxidizing agent, thereby volatilizing the arsenic content and obtaining a metallic bismuth and bismuth oxide substantially free from lead and arsenic.

5. The method for recovering bismuth from a metallurgical substance containing the same along with lead and arsenic which comprises treating said metallurgical substance with dilute nitric acid in a quantity of approximately 10 to 12% in excess of that necessary to convert the lead content into lead nitrate, agitating said acid solution until the lead is dissolved in the form of a nitrate and a minor portion of the bismuth content is precipitated as bismuth subnitrate, and subjecting the residua1 material containing metallic bismuth, bismuth subnitrate and arsenic to an oxidizing fusion wherein said bismuth subnitrate is the oxidizing agent, whereby the arsenic content is volatilized and the resultant material comprises a slag of bismuth oxide and molten metallic bismuth.

6. The method for recovery of substantially pure bismuth from a metallurgical substance containing the same along with lead and arsenic which comprises treating said metallurgical substance in ground condition with a quantity of dilute nitric acid slightly in excess of that theoretically necessary to convert all of the lead to nitrate solution, stirring the mixture for a time interval sufficient to dissolve said lead content and to precipitate a minor portion of the bismuth content as a subnitrate of bismuth, separating the residual material containing bismuth, bis- ,muth subnitrate and arsenic, subjecting said 

